1. Field of the invention
The invention relates to an optical pickup system that irradiates a light beam from a light source to an object through a half mirror and introduces the light beam reflected by the object to an optical detector through the half mirror.
2. Prior art
Optical pickup systems are designed to produce astigmatism of an arbitrary magnitude in a predetermined direction in order to detect a focusing error of an objective lens for an object, and effect focusing control of the objective lens by utilizing the fact that the section of the luminous flux is substantially circular at the focal point. Known as a means for producing astigmatism are a method of disposing a cylindrical lens along an optical path and a method of arranging a half mirror so as to be inclined relative to the optical axis. Exemplary methods of arranging a half mirror so as to be inclined relative to the optical axis are disclosed in Japanese Patent Unexamined Publications Nos. 202339/1986 and 302548/1989. These methods are designed to correct a coma since the coma is produced together with an astigmatism when the half mirror is arranged so as to be inclined relative to the optical axis. The methods disclosed in the above-described publications will be outlined below.
The method proposed in the above publication No. 202339/1986 is such that the half mirror is arranged so as to be inclined relative to the optical axis and that another optical member is arranged so as to be inclined relative to the optical axis. This is a design to correct the coma by the interaction of the half mirror and the other optical member. The half mirror and another optical member are formed of parallel flat plates or concave or convex surfaces.
FIGS. 3 and 4 outline the above publication No. 302548/1989; and FIG. 4 is a side view as viewed from a side of a totally reflecting mirror 4 in FIG. 3. In FIGS. 3 and 4, a laser beam from a laser light source 1 passes through a diffraction grating 2, is reflected by a half mirror 3, and converges onto an optical disk 6, which is a symmetrical object, by an objective lens 5 via the totally reflecting mirror 4. The reflected beam from the optical disk 6 reaches an optical element 8 after having passed through the objective lens 5 and the mirror 4, passing through the half mirror 3, and passing through a correcting lens 7. The correcting lens 7 is made of a glass or plastic material, and has an anisotropically curved surfaces R1 and R2 on the half mirror 3 side and the optical element 8 side, respectively. The anisotropically curved surface R1 is such a cylindrical surface as to produce astigmatism in a direction of cancelling out the astigmatism produced by luminous flux having passed through the half mirror 3. The other anisotropically curved surface R2 is such a toric or toroidal surface as to produce astigmatism in an arbitrary direction and set the magnification of imaging on the surface of the optical element 8 to a predetermined value. In order to cancel out the effect of the coma produced by the half mirror 3, an optical axis S--S of the correcting lens 7 is inclined by .PHI. relative to an optical axis X--X.
According to the method disclosed in the publication No. 202339/1986, it is so designed that a coma produced by the half mirror disposed so as to be inclined relative to the optical axis is cancelled out by a coma produced by the parallel flat plate or lens arranged so as to be inclined relative to the optical axis. Thus, in order to effectively cancel out the coma produced by the half mirror, the angle of inclination of the optical element such as the parallel plate or lens must be large, and as a result the optical element is difficult to fabricate with the dimension thereof in the direction of the optical axis becoming large. Further, since the angle of inclination of the optical element must be large relative to the mounting frame, thereby making the dimensional control difficult when the optical element is made by resin molding or the like. Still further, since the angle of inclination of the optical element relative to the optical axis is large, the optical path for the light beam transmitting the optical element becomes long, which imposes the problems such as being disadvantageous in downsizing the optical pickup system, making the loss of light large, or greatly impairing the transmitted light due to inconsistency of the media.
Also in the method proposed in the above-described publication No. 302548/1989, the optical axis of the correcting lens 7 must be inclined in accordance with the curvature of the correcting lens 7 in order to cancel out the effect of a coma produced by the half mirror 3. Depending on the design condition of the optical system, the angle of inclination of the correcting lens 7 must be large in order to cancel out a coma produced by the half mirror 3 effectively. Thus, the same problem is encountered by the method proposed in the publication No. 202339/1986.